1. Technical Field of the Invention
The embodiments of the invention relate to communication devices and more particularly to distortion cancellation in a radio receiver.
2. Description of Related Art
Various wireless communication systems are known today to provide links between devices, whether directly or through a network. Such communication systems range from national and/or international cellular telephone systems, the Internet, point-to-point in-home system, as well as other systems. Communication systems typically operate in accordance with one or more communication standards or protocol. For instance, wireless communication systems may operate using protocols, such as IEEE 802.11, Bluetooth, advanced mobile phone services (AMPS), digital AMPS, global system for mobile communications (GSM), code division multiple access (CDMA), local multi-point distribution systems (LMDS), multi-channel-multi-point distribution systems (MMDS), as well as others.
For each wireless communication device to participate in wireless communications, it generally includes a built-in radio transceiver (i.e., receiver and transmitter) or is coupled to an associated radio transceiver (e.g., a station for in-home and/or in-building wireless communication networks, modem, etc.). Typically, the transceiver includes a baseband processing stage and a radio frequency (RF) stage. The baseband processing provides the conversion from data to baseband signals for transmitting and baseband signals to data for receiving, in accordance with a particular wireless communication protocol. The baseband processing stage is coupled to a RF stage (transmitter section and receiver section) that provides the conversion between the baseband signals and RF signals. The RF stage may be a direct conversion transceiver that converts directly between baseband and RF or may include one or more intermediate frequency stages.
Furthermore, wireless devices typically operate within certain radio frequency ranges or band established by one or more communications standards or protocols. A local oscillator generally provides a local oscillation signal that is used to mix with received RF signals or baseband signals that are to be converted to RF signals in the modulation/demodulation stage of the RF front end. A synthesizer may be used to set the frequencies to drive the local oscillator to provide the desired frequencies for mixing, in which the desired frequencies are generally based on the channel frequencies established for the particular standard or protocol.
In a typical RF receiver, a basic functionality is for the receiver to convert a received signal on a specific channel, having a designated carrier frequency and bandwidth, into a baseband signal, having a stream of baseband samples that resembles the complex baseband representation of the signal transmitted on the specific channel. The data is then recovered from the baseband signal. In operation, the bandwidth limiting properties of the receiver is capable of attenuating or removing many of the unwanted signals. However, some unwanted signals are within the receiving bandwidth of the receiver and are coupled via the antenna to the receiver front end. In some instances, these unwanted signals are due to the transmitted signal having the same or close carrier frequency to the received signal. In some instances, the unwanted signals may be from nearby channels (e.g. adjacent channels). In some instances, the unwanted signals may be the result of interfering signals occurring with the allowed bandwidth. In other instances, the interference may be from noise. It may also be from other sources or a combination of these sources. Any strong unwanted signal that is coupled through to the receiver front end is referred to as a “blocker” or “blockers.”
In an ideal receiver, which would be perfectly linear in its response characteristics, only the wanted (desired) signal will be coupled through and all unwanted signals would be rejected. However, in practical applications, ideal receivers are difficult or impossible to achieve. Hence, because of the non-linear response characteristics of the receiver, a blocker signal may interact with the desired signal. In some instances, the blocker signal may mix with local signals, such as the local oscillator signal, to inject interfering or inter-modulation signal(s) on top of the desired signal. Those interfering signals that are outside of the specified bandwidth may be attenuated, but others may be present within the bandwidth to interfere or interact with the desired signal. One technique of dealing with these blockers is to provide complex filters to remove or cancel the blockers. Still, other techniques may utilize more complex cancellation techniques. However, utilizing complex filters may require specialized filtering circuitry (sometimes located off chip). These and other cancellation techniques are generally frequency dependent, which makes it difficult to obtain the cancellation estimates, because the blockers may shift in frequency/phase. Further, these complex methods may add significant cost to the design and/or manufacture of the receiver.
Accordingly, there is a need to obtain a more desirable way to cancel the effects of a blocker signal that is more independent of frequency in estimating the cancellation values.